1. Field of the Invention
This invention relates to lens position control devices in optical instruments such as cameras, observation instruments, etc.
2. Description of the Related Art
The general form of a zoom lens (which has found a use in video cameras) is comprised of fur lens groups as shown in FIG. 9.
In FIG. 9, the first lens group 1 arranged at the front of a lens barrel functions as a focusing lens (F). The second lens group 2 is the variator (V) for varying the focal length of the entire system. The third lens group 3 is the compensator (C) for bringing a plane of sharp focus to the proper position after the focal length varying operation. The fourth lens group 4 is the relay lens for forming an image of an object. Incidentally, FIG. 9 shows that the zoom lens set at the wide-angle end (the shortest focal length) and focused on an object at infinity. In the following, for explanation of the way in which each lens group moves, the positions of the lens group 1 (F), the lens group 2 (V), and the lens group 3 (C) in this state are considered here as respective zero (0) positions.
FIGS. 10(A) and 10(B) to FIG. 12 show the relationships of the variations of the positions of the lens groups 1, 2 and 3 (F, V and C) with the focal length of that zoom lens or the object distance. By reference to these figures, the features of that zoom lens are described below.
FIG. 10(A) is a graph of the position to which the lens group 2 (V) has moved along an optical axis taken in the abscissa and the focal length f of the zoom lens taken in the ordinate, illustrating how the focal length f varies when the lens group 2 (V) moves. Incidentally, W represents the wide-angle position where the focal length of the zoom lens becomes shortest, and T represents the telephoto position where the focal length of the zoom lens becomes longest.
FIG. 10(B) is a graph of the axial position of the lens group 3 (C) taken in the abscissa and the focal length f of the zoom lens taken in the ordinate, illustrating the variation of the focal length f with the variation of the position of the lens group 3 (C).
FIG. 11 is a graph of the reciprocal of the distance (in meters) to an object to be photographed taken in the abscissa and the position to which the lens group 1 (F) is moved forward along the optical axis taken in the ordinate, illustrating the variation of the object distance with the variation of the position of the lens group 1 (F).
FIG. 12 is a graph of the position to which the lens group 1 (F) is moved forward along the optical axis taken in the ordinate and the focal length f of the zoom lens taken in the abscissa, illustrating the relationship between the position of the lens group 1 (F) and the focal length f and exemplifying a number of positions of the lens group 1 (F) about the respective cases where the distance to the object is 1 m, 2 m, 3 m, or infinity.
It is understoqd from these graphs that the publicly known zoom lens has the following features. That is, as is apparent from FIG. 11 and FIG. 12, in a case where the object distance does not change, even when the focal length is changed by zooming, there is no need to move the lens group 1 (F). Therefore, the lens group 2 (V) and the lens group 3 (C) may be interlocked with each other according to the characteristic curves of FIGS. 10(A) and 10(B). Therefore, the position of each lens group can be relatively easily controlled. Thus, there is a merit that its position control can be carried out by a mechanical control mechanism such as a cam.
FIG. 13 is a view illustrating the interlocking mechanism of the lens group 2 (variator lens) and the lens group 3 (compensator lens) of the publicly known zoom lens. In FIG. 13, a lens group holding frame 5 holding the lens group 2 (V) and another lens group holding frame 6 holding the lens group 3 (C) are guided along the optical axis by guide bars 7 and 8. A cam tube 9 has camming slots 9a and 9b into which pins 5a and 6a mounted on the lens group holding frames 5 and 6 are inserted respectively. A fixed tube 10 is fitted on the outer diameter of the cam tube 9 and fixedly secured to a stationary member such as lens barrel. A zoom actuating ring 11 is fixed to the cam tube 9 by a connector 11a and fitted on the outer diameter of the fixed tube 10 only rotatably relative thereto. During zooming, the zoom actuating ring 11 is rotated, which in turn rotates the cam tube 9. As a result, the relative position of the pin 5a in the camming slot 9a and the relative position of the pin 6a in the camming slot 9b change so that the lens group holding frame 5 and the lens group holding frame 6 are moved along the axial axis relative to each other.
However, the prior known control mechanism using the cam tube has also a disadvantage in that the fitting tolerances of the cam tube and the machining tolerance of the camming slots have to be made very severe and, therefore, the production cost is very high.
Moreover, as is apparent from FIG. 11 and FIG. 12, in the conventional zoom lens, to focus on the object at a closest distance (for example, 1 m or less), the total focusing movement of the lens group 1 (F) must be increased in proportion to the inverse number of the distance. As the minimum object distance decreases, the total focusing movement increases infinitely. Hence, there is a fundamental drawback that close-up photography at the minimum object distance is impossible.
On this account, in recent years, a zoom lens in which one of the lens groups other than the lens group 1 (F) is moved for focusing, i.e., a zoom lens of the so-called inner focus type, is proposed.
An example of this zoom lens, as shown in FIG. 14, though including the lens group 1 and the lens group 2, lacks the lens group 3 equivalent to the conventional compensator. In this zoom lens, while the lens group 1 and the front lens 4A (R) of the lens group 4 are arranged to be stationary, the lens group 2 (variator) is arranged likewise as in the publicly known zoom lens of FIG. 9 to move when the focal length is varied. Also, the rear lens 4B (RR) of the relay lens group 4 has functions of focus adjustment and compensation. By making the rear lens 4B move along the optical axis likewise as the conventional compensator lens, focus adjustment and compensation are performed.
Another example of construction and arrangement of the inner focus type zoom lens is shown in FIG. 17. In this case, four lens groups are in use and the lens group 2 has the function of varying the focal length likewise as the conventional 4-group zoom lens of FIG. 9. However, what is different in comparison with FIG. 9 is that the lens group 1 is fixedly secured to the fixed lens barrel 101. For this reason, the lens group 3 which would do only the compensating work in the prior art, has to have the focusing function.
In the zoom lens having such a construction and arrangement of the lens groups, because the lens group 1 is arranged not to move, focusing can be effected even to very short distances, but, because the relative positional relationship between the movable lens groups, i.e., the lens group 2 and the rear relay lens 4B of FIG. 14, or the lens group 3 of FIG. 17, is very complicated, such a cam mechanism or other simple control mechanism as shown in FIG. 13 does not suffice for controlling the differential movement of the lens group 2 and the rear relay lens 4B of FIG. 14 or the lens group 3 of FIG. 17. Therefore, it is very difficult to realize the zoom lens of the form shown in FIG. 14 or FIG. 17, so long as the mechanical mechanism only is available.
FIG. 15 is a graph of the position of the lens group 2 (V) of the zoom lens of FIG. 14 taken in the abscissa and the position of the rear relay lens 4B (RR) taken in the ordinate, illustrating the relative positional relationship of both lenses in discrete values of the object distance. As is apparent from FIG. 15, because the relative positional relationship of both lenses varies as the distance of the object varies from infinity to progressively smaller values of 3 m, 1 m, 0.5 m, 0.2 m, 0.01 m, it is understandable that with a simple control mechanism such as the cam, it is impossible to control both lenses.
Recently, however, there has been made a proposal for employing a control method that depending on the result of detection of whether or not the sharp image is formed at a right place or focal plane, the rear relay lens 4B only is controlled relative to the lens group 2, so that the zoom lens of FIG. 14 is realized. There has also been announced a commodity developed on the basis of this proposal.
FIG. 16(A) is a schematic view illustrating the lens position control method and the lens form employed in that proposal and commodity. The zoom lens includes a lens group 1, a lens group 2, a front lens 4A of a relay lens group 4, a rear lens 4B of the relay lens group 4, a detecting means 12 for detecting an image formed on the focal plane, a focus control (AF) circuit 13 for detecting when the image is in focus, and for controlling focusing, and a drive means 14 controlled by the AF circuit 13 to perform position determination and driving of the rear relay lens 4B.
FIG. 16(B) to FIG. 16(D) show an example of the automatic focus adjusting device. In FIG. 16(B), an entire picture area 17 of the video camera contains therein a spot 18 from which a signal for distance measurement is taken out. Also, an actual object is assumed to have a contrast 19. In FIG. 16(C), part (a) is that fragment of this contrast which enters the spot 18. Then, (b) is a Y signal output, (c) represents the differentiated value of the Y signal, (d) is its absolute value, and (e) is the peak-held signal, where the height A represents the degree of focus (hereinafter called the "blur evaluation value"). FIG. 16(D) is a graph with the abscissa in the position of the lens group 1 of FIG. 9 or the lens 4B of FIG. 14, and the ordinate in the blur evaluation value A. The in-focus state comes out at the position B of the peak.
As another or improved method, Japanese Laid-Open Patent Application No. Sho 62-296110, Japanese Laid-Open Patent Application No. Sho 62-284316, etc. have been proposed. This method memorizes the unit movement amount of the lens having both the function of the compensator and the focusing function (hereinafter called the "double-purpose" lens) in correspondence with the predetermined movement amount of the variator lens according to either the position information of the variator lens and the double-purpose lens or the position information of the variator lens and the distance operating member (distance ring), so that each time the variator lens is moved by a predetermined amount, the movement of the double-purpose lens is controlled on the basis of the memorized unit movement amount.
By the way, in the publicly known zoom lens and lens position control method shown in FIG. 16(A), if the accuracy and speed of the input signal from the image detecting means 12 to the AF circuit 13 are high, the image formed on the focal plane would not be blurred or distorted. In fact, however, by the response delay etc. due to the cycle of distance measurement etc., the possibility of lowering the accuracy of control of the rear relay lens 4B is very high. Hence, there is a serious drawback that a large blurring is liable to occur.
Also, in the above-described improved method, because the detection of the predetermined amount of movement of the variator lens becomes the premise, in order to obtain a highly accurate movement of the aforesaid double-purpose lens, there is need to make go extremely fine the amount of movement of the variator lens. Further, the moving speed of this double-purpose lens must be increased. Otherwise, it would take a considerably long time to correct the produced blur.